Uranium(VI) and thorium(IV) extraction by malonamides: Impact of ligand molecular topology on selectivity

Abstract

Uranium(VI) and thorium(IV) extraction from nitrate media with N,N′-dimethyl,N,N′-dibutylteradecylmalonamide (DMDBTDMA) and N,N,N′,N′-tetrahexylmalonamide (THMA) in toluene has been investigated. These two diamides are structural isomers, differing only in the position of alkyl chain substituents. With both ligands, the extraction of uranium(VI) and thorium(IV) is more efficient at higher acidity while uranium(VI)/thorium(IV) selectivity is higher at lower acidity, particularly with DMDBTDMA, for which an almost selective extraction of uranium(VI) has been observed. The selectivity difference between the two ligands has been proven to be mainly due to thorium(IV) extraction, which is strongly dependent on the malonamide molecular structure. On the other hand, uranium(VI) extraction follows the same trend with both DMDBTDMA and THMA. In-depth analysis of molecular and supramolecular features of extracting organic solutions suggests that the disparity of thorium(IV) extraction trends between the two malonamides can be linked to the difference in thorium(IV)-ligand interactions: thorium(IV) extraction appears to lead to rather stable classical coordination complexes with THMA, and more complex supramolecular species with DMDBTDMA. Altogether, these results suggest the prevalence of hydrophobic driven interactions for extraction of metallic cations with DMDBTDMA and electrostatic driven interactions in the case of THMA.